Balanced ultrasonic blade including a plurality of balance asymmetries

ABSTRACT

A balanced ultrasonic surgical instrument is described wherein the balanced ultrasonic surgical instrument includes an ultrasonic transmission rod and an ultrasonically actuated blade attached to the distal end of the ultrasonic transmission rod. According to the present invention, the ultrasonically actuated blade includes a treatment portion and a balance portion. The treatment portion has a functional feature such as, for example, a curved blade which makes the treatment portion asymrnetric. The balance portion includes at least first and second asymmetric balance features designed and positioned to balance out any undesirable torque generated by the treatment portion. The balance portion further extends generally from a node point on the ultrasonic transmission rod to the proximal end of the treatment portion.

This is a continuation of application Ser. No. 09/499,989 Filed Feb. 8,2000 now U.S. Pat. No. 6,328,751; which is a continuation of applicationSer. No. 09/106,686 Filed Jun. 29, 1998 now abandaoned.

This application is related to the following co-pending patentapplications: application Ser. Nos. 09/106,028; 09/106,415; and09/106,661 which are hereby incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates, in general, to curved ultrasonic bladesfor use in surgical instruments and, more particularly, to balancedcurved ultrasonic blades including two or more balance asymmetries.

BACKGROUND OF THE INVENTION

Ultrasonic instruments are often used in surgery to cut and coagulatetissue. Exciting the end effector (e.g. cutting blades) of suchinstruments at ultrasonic frequencies induces longitudinal vibratorymovement which generates localized heat within adjacent tissue,facilitating both cutting and coagulation. Because of the nature ofultrasonic instruments, a particular ultrasonically actuated endeffector may be designed to perform numerous functions, including, forexample, cutting and coagulation. The structural stress induced in suchend effectors by vibrating the blade at ultrasonic frequencies may havea number of undesirable effects. Such undesirable effects may include,for example, substantial transverse motion in the instrument waveguidewhich may lead to, for example, excess heat generation in the waveguideor premature stress failure. The undesirable effects of vibrating asurgical end effector at ultrasonic frequencies are compounded where theend effector is not symmetrical, that is, where the mass of the endeffector is not distributed symmetrically about a line extending throughthe central axis of the transmission waveguide. Therefore, one way toimprove the performance of ultrasonically actuated end effectors is todesign end effectors which are substantially symmetric about the centralaxis of the transmission waveguide. Alternatively, the surgical endeffector may be small and short, in which case the end effector will actlike a small lumped mass at the end of the transmission waveguide andwill not induce substantial transverse motion in the waveguide. Where itis desirable to design end effectors which are not symmetric,performance may be improved by designing the end effector such that thecenter of mass of the end effector is located along a line which extendsthrough the central axis of the waveguide. One known method of movingthe center of mass is to add or subtract mass opposite or close to theasymmetric region until the center of mass lies along a central axis. Asa further alternative, longitudinal vibratory motion in the waveguidemay be reduced or eliminated by using thicker, more robust waveguideswhich are not as subject to transverse vibratory motion. However, theuse of thick waveguides may not be an acceptable alternative where theultrasonic surgical instrument is being designed for use in minimallyinvasive surgery such as endoscopic or laparoscopic surgery. In suchinstruments it is generally desirable to reduce the diameter of theultrasonic waveguide in order to fit the instrument through the tinysurgical incisions, narrow body orifices and through trocars presentlybeing used and being designed for future procedures. Long thinultrasonic waveguides, such as those used in instruments for minimallyinvasive surgery, are particularly susceptible to transverse vibrationsintroduced by imbalances in the end effector.

For certain applications, it is desirable to include one or more axiallyasymmetrical features, (e.g. blade curvature) to enhance performance ofthe end effector. It may also be desirable to design such end effectorsto be relatively long, in order to facilitate certain surgicalprocedures. In such end effectors, it is not always possible ordesirable to include opposed balancing features in the treatment portionin order to balance the end effector by aligning the center of mass withthe central axis of the transmission waveguide. It would, therefore, bedesirable to design an ultrasonic surgical instrument including awaveguide and an ultrasonic end effector wherein undesirable transversevibrations resulting from the inclusion of beneficial asymmetricalfeatures (e.g. a long curved blade) in the working portion of the endeffector have been reduced or eliminated. It would further beadvantageous to design such an instrument wherein the undesirabletransverse vibrations have been reduced or eliminated without addingbalancing features to the treatment portion of the end effector. Itwould further be advantageous to design an end effector whereinundesirable transverse vibrations resulting from the inclusion ofbeneficial asymmetrical features in the treatment portion of the endeffector have been reduced or eliminated by adding asymmetricalbalancing features proximal to the treatment portion of the endeffector. It would further be advantageous to design an asymmetric endeffector with a center of mass which is not on the central axis of thetransmission wave guide wherein significant transverse motion is notinduced in the waveguide by the asymmetric end effector.

SUMMARY OF THE INVENTION

A balanced ultrasonic surgical instrument including an ultrasonictransmission rod and an ultrasonically actuated blade attached to thedistal end of the ultrasonic transmission rod. According to the presentinvention, the ultrasonically actuated blade includes a treatmentportion and a balance portion. The treatment portion has a functionalfeature such as, for example, a curved blade which makes the treatmentportion asymmetric. Such a functional feature may be referred to as afunctional asymmetry. The balance portion includes at least first andsecond asymmetric features designed and positioned to balance out anyundesirable torque generated by the treatment portion. Such balancefeatures may be referred to as balance asymmetries and may includeasymmetric features such as, for example, notches, flats, bumps orraised regions. In an ultrasonic instrument according to the presentinvention, the balance portion generally extends from a node point onthe ultrasonic transmission rod to the proximal end of the treatmentportion. In an ultrasonic surgical instrument according to the presentinvention, the first and second balance asymmetries are positioned suchthat transverse vibrations in the ultrasonic transmission rod aresubstantially reduced and may be reduced to approximately zero. Further,in an ultrasonic surgical instrument according to the present invention,the balance ratio of the transmission waveguide may be reduced to lessthan 1:10 and may be further reduced to less than 1:200.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe appended claims. The invention itself, however, both as toorganization and methods of operation, together with further objects andadvantages thereof, may best be understood by reference to the followingdescription, taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is an exploded perspective view of an ultrasonic surgicalinstrument according to the present invention.

FIG. 2 is a side view of the distal end of an ultrasonic transmissionassembly according to the present invention.

FIG. 3 is a top view of the distal end of an ultrasonic transmissionassembly according to the present invention.

FIG. 4 is a perspective view of the distal end of an alternateembodiment of an ultrasonic transmission assembly according to thepresent invention.

FIG. 5 is a perspective view of the distal end of the ultrasonictransmission assembly shown in FIG. 4 with a phantom x,y plane drawnthrough the center of the ultrasonic transmission waveguide.

FIG. 6 is a perspective view of the distal end of the ultrasonictransmission assembly shown in FIG. 4 with a phantom x,z plane drawnthrough the center of the ultrasonic transmission waveguide.

FIG. 7 is a side view of an alternate embodiment of the distal end ofthe ultrasonic transmission assembly shown in FIG. 4.

FIG. 8 is a top view of the distal end of the ultrasonic transmissionassembly shown in FIG. 7.

FIG. 9 is a perspective view of the distal end of the ultrasonictransmission assembly shown in FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is an exploded perspective view of an ultrasonic surgicalinstrument 10 according to the present invention. In FIG. 1, ultrasonicend effector 12 is mechanically coupled to ultrasonic transmissionwaveguide 14 to form ultrasonic transmission assembly 11. Ultrasonictransmission waveguide 14 is positioned in outer sheath 16 by mountingo-ring 18 and sealing ring 20. One or more additional dampers or supportmembers (not shown) may also be included along ultrasonic transmissionwaveguide 14. Ultrasonic transmission waveguide 14 is affixed to outersheath 16 by mounting pin 21, which passes through mounting holes 23 inouter sheath 16 and mounting slot 25 in transmission waveguide 14.

FIG. 2 is a side view of the distal end of ultrasonic transmissionassembly 11, including end effector 12. FIG. 2 further includes anordinate system in which: the x-axis lies along central axis 24 ofultrasonic transmission waveguide 14 while the y-axis is the axis ofcurvature of treatment region 26. In the embodiments of the inventiondescribed herein, end effector 12 is affixed to the distal end oftransmission waveguide 14 at balance node 22. Central axis 24 oftransmission waveguide 14 extends from the proximal end of transmissionwaveguide 14 to the distal end of transmission waveguide 14.Transmission waveguide 14 is symmetrical about central axis 24. Endeffector 12 includes treatment region 26, which is located at the distalend of end effector 12 and balance region 28 which is located betweentreatment region 26 and balance node 22. Treatment region 26, beingcurved, includes two surfaces, a concave top surface 30 and a convexbottom surface 32. Convex bottom surface 32 is substantially planar orflat along the y-axis of the blade. Treatment region 26 further includesrounded tip 34. In the illustrated embodiment of the invention, balanceregion 28 includes a first cutout 38 and a second cutout 40 which act asasymmetric balance features. First cutout 38 extending from the proximalend of concave surface 30 to a first predetermined point 42 which isdistal to balance node 22. Second cutout 40 extends from the proximalend of convex surface 32 to a second predetermined point 44 which isdistal to point 42 and balance node 22.

FIG. 3 is a top view of the distal end of ultrasonic transmissionassembly 11, including end effector 12. In FIG. 3, blade edges 36 arepositioned on both sides of treatment region 26 and extend from theproximal end of treatment region 26 to rounded tip 34. Theintersection-of concave surface 30 and convex surface 32 form bladeedges 36. Central ridge 37 runs from the distal end of balance region 28to rounded tip 34 along the center of treatment region 26. Central ridge37 forms a portion of concave top surface 30. Central ridge 37 addsstrength, stiffness and rigidity to treatment region 26 by givingtreatment region 26 a substantially trapezoidal cross-section.

FIG. 4 is a perspective view of the distal end of an embodiment of anultrasonic transmission assembly 11. FIG. 5 is a perspective view of thedistal end of ultrasonic transmission assembly 11 of the embodiment ofthe invention shown in FIG. 4 with a phantom x,y plane 52 drawn throughthe center of ultrasonic transmission waveguide 14. In FIG. 5, phantomx,y plane 52 passes through central axis 24. Since treatment region 26curves away from x,y plane 52, end effector 12 is not symmetrical aboutx,y plane 52. Plane 52 may, therefore, be referred to as the plane ofasymmetry for end effector 12.

FIG. 6 is a perspective view of the distal end of the ultrasonictransmission assembly 11 of the embodiment of the invention shown inFIG. 4 with a phantom x,z plane 50 drawn through the center ofultrasonic transmission waveguide 14. In FIG. 6, phantom x,z plane 50passes through central axis 24 at an angle at 90° to x,y plane 52. Endeffector 12 is substantially symmetrical about x,z plane 50. Plane 50may, therefore, be referred to as the plane of symmetry for end effector12. FIG. 7 is a side view of an alternate embodiment of the distal endof the ultrasonic transmission assembly shown in FIG. 4. In theembodiment of the invention illustrated in FIG. 7, end effector 12 hassubstantially the same shape and structure as the embodiment of theinvention illustrated in FIGS. 1-7 except the embodiment of FIG. 7includes sharp tip 35 at the distal end of treatment region 26. FIG. 8is a top view of the distal end of the ultrasonic transmission assemblyshown in FIG. 4. FIG. 9 is a perspective view of the distal end of theultrasonic transmission assembly shown in FIG. 4.

Ultrasonic surgical instrument 10 has a treatment region 26 whichincludes a curved blade designed to cut and coagulate tissue whenvibrated at ultrasonic frequencies, such as, for example, fifty-fivekilohertz (55 kHz). Treatment region 26 is curved to provide the surgeonwith better access and visibility when using ultrasonic instrument 10.As illustrated in FIGS. 5-6, curved treatment region 26 is symmetricalabout x,z plane 50 but is not symmetrical about x,y plane 52. Whentreatment region 26 is vibrated at an appropriate ultrasonic frequencyto facilitate cutting and coagulation, the asymmetrical shape oftreatment region 26 will tend to induce undesirable forces, includingtorque, which are transmitted back to transmission waveguide 14 andinduce undesirable transverse vibrations in transmission waveguide 14.

As previously described, it is known that these undesirable transversevibrations may be minimized and the end effector balanced by designingthe end effector such that the center of mass at any point along the endeffector is positioned on or very near the central axis of thetransmission waveguide. However, where, as in the present invention, theasymmetry (e.g. the curve of treatment region 26), causes the center ofmass to diverge substantially from a line extending from the centralaxis of the transmission waveguide and the addition of balance featureswithin the treatment region is undesirable, the blade must be balancedusing an alternative method.

According to the present invention, end effector 12 is balanced byreducing or eliminating the torque induced in end effector 12 proximalto treatment region 26 as a result of including functional asymmetricalfeatures in treatment region 26. A convenient physical point ofreference at the proximal end of end effector 12 is balance node 22. Itshould be noted that balance node 22 may be any node of longitudinalvibration along transmission waveguide 14 and is not necessarily themost distal vibratory node. Nodes of longitudinal vibration occur athalf wavelength intervals along the transmission waveguide, wherein thewavelength of interest is the wavelength of the frequency at which theultrasonic end effector is driven (e.g. 55 kHz). In the embodiment ofthe invention illustrated in FIG. 3, the asymmetric functional featurescomprise curved treatment region 26 having rounded tip 34. A feature isasymmetric when its cross-section is not symmetric with respect towaveguide central axis 24. A feature is symmetric when the cross-sectionis symmetric with respect to waveguide central axis 24. That is, afeature is symmetric when a chord through a cross-section of the portionof the end effector, which includes the feature, is bisected by centralaxis 24.

According to the present invention, a balance region 28 is included inend effector 12 and end effector 12 is balanced by positioning at leasttwo asymmetric balance features in balance region 28 between theproximal end of treatment region 26 and balance node 22. The size, shapeand position of the asymmetric balance features included in balanceregion 28 are selected to reduce the torque at a balance point 29 tozero or as close to zero as possible. Balance point 29 is on centralaxis 24 positioned at, for example, balance node 22. The degree to whichtorque is reduced will depend upon particular design and manufacturingconstraints. Thus, by appropriately arranging asymmetric balancefeatures in balance region 28, the torque induced by the asymmetricfunctional features in treatment region 26 may be canceled by the torqueinduced by the asymmetric balance features. With the summation of torquedistal to end effector 12 minimized, the transverse vibration induced intransmission waveguide 14 will be substantially reduced and may bereduced to approximately zero.

In order to determine whether an asymmetric end effector has beenproperly balanced, it may be appropriate to measure the torque inducedin transmission waveguide 14. The relative magnitude of the torqueinduced in transmission waveguide 14 may be estimated by taking theratio of the peak lateral displacement, less Poisson's swelling (alsoreferred to as longitudinal node swelling), at any transverse vibratoryantinode of the transmission wave guide to the peak longitudinaldisplacement at any longitudinal vibratory antinode of the transmissionwaveguide. The closer the ratio is to zero, the less transversevibration is being induced in the waveguide. Thus, the ratio of peaklateral displacement to peak longitudinal displacement may be referredto as the “balance ratio”. In one embodiment of the present invention, ablade would be considered balanced if the balance ratio of peak lateraldisplacement to peak longitudinal displacement is 1:10 or less. Moreparticularly, using the techniques described herein, it may be possibleto achieve balance ratios of 1:200 or less.

An asymmetric feature is a feature of the end effector wherein thecenter of mass of the feature is off a line extending from the centralaxis of the transmission waveguide. In an end effector having asymmetrical orientation and an asymmetrical orientation, such as the endeffector illustrated in the Figures, all of the asymmetric features arein a plane parallel to the plane of symmetry.

The mass and shape of the asymmetric balance features introduced intobalance region 26 are determined by a number of factors. The torqueinduced at balance point 29 is equal to the integral over volume of thecross product of the vector acceleration at each point on the endeffector with a position vector multiplied by a density scalar. Thedensity scaler is a function which represents the density of the endeffector at each point on the end effector. Expressing that equationmathematically, the torque ({tilde over (T)}) at balance point 29 is$\begin{matrix}{{\int_{x_{0}}^{x_{1}}{\int_{y_{0}}^{y_{1}}{\int_{z_{0}}^{z_{1}}{{\overset{\rightharpoonup}{A}\left( {x,y,z} \right)} \times {\overset{\rightharpoonup}{o}\left( {x,y,z} \right)}{\rho \left( {x,y,z} \right)}{z}{y}{x}}}}},} & (1)\end{matrix}$

where:

x₀, y₀, and z₀ are located in the plane x=0 at balance point 29;

x₁, y₁, and z₁ are located in a plane tangential to the distal tip ofend effector 12 and, with x₀, y₀, and z₀, define a region which enclosesend effector 12;

{right arrow over (A)}(x, y, z) is the acceleration of the blade at anypoint (x,y,z);

{right arrow over (o)}(x, y, z) is a vector indicative of the positionof the point (x,y,z) with respect to balance point 29; and

ρ(x, y, z) is the density of the blade at any point (x,y,z).

Therefore, in a balanced end effector designed according to the presentinvention, an end effector 12 is first designed which incorporates oneor more beneficial asymmetries in treatment region 26 (e.g. curved bladeedges 36). A balance node point is then selected at a convenientvibratory node along waveguide 14. Normally the balance node point willbe the most distal vibratory node on waveguide 14. A symmetrical (e.g.cylindrical) balance region 28 is then incorporated into end effector12. In the illustrated embodiments, balance region 28 extends frombalance node 22 to the proximal end of treatment region 26. Normally theproximal end of treatment region 26 will correspond with the proximalend of the proximal most beneficial asymmetry. For example, in theembodiment of the invention illustrated in FIG. 2, the proximal end oftreatment region 26 corresponds to the proximal end of curved blade edge36. Once the appropriate beneficial asymmetries have been designed intothe end effector, the torque induced at balance point 29 by the endeffector design, including beneficial asymmetries. may be calculatedusing Equation (1) above.

In using Equation (1) above to calculate the torque induced by anyparticular asymmetry at balance point 29, a suitable first step is tofind a mathematical expression for {right arrow over (A)}(x, y, z), theacceleration at each point along end effector 12, along with amathematical expression for ρ(x, y, z), the density at each point alongend effector 12, and a mathematical expression for {right arrow over(o)}(x, y, z), the position vector for each point along end effector 12with respect to balance point 29. For convenience, {right arrow over(o)}(x, y, z) may be referred to as the offset vector. As Equation (1)indicates, the torque induced at balance point 29 by end effector 12 isthe volume integral of the cross product of the acceleration vector withthe product of the offset vector and scalar density. In Equation (1),the integral is taken over the volume of the end effector. Generallystated, the torque induced at balance point 29 will be equal to the sumof the torques induced by each asymmetry in end effector 12. Thus anoptimum design may be obtained where balance asymmetries areincorporated into balance region 28 such that the torque induced by thebalance asymmetries cancel the torque induced by the beneficialasymmetries.

In an ideal situation it would be possible to express {right arrow over(A)}(x, y, z), {right arrow over (o)}(x, y, z), and ρ(x, y, z) usingmathematical formulas which could be conveniently integrated over thevolume of end effector 12. However, it is generally very difficult todevelop such mathematical formulas for ultrasonic surgical end effectorgeometry because ultrasonic surgical end effectors do not generallyassume continuous geometric shapes such as cones or cylinders.Therefore, once the variables have been calculated or modeled, theintegral may be calculated using, for example, a numerical integrationprogram. Of the parameters {right arrow over (A)}(x, y, z), {right arrowover (o)}(x, y, z), and ρ(x, y, z), the most difficult to calculate isgenerally the acceleration vector {right arrow over (A)}(x, y, z) foreach point along end effector 12, particularly for end effectors havingcomplex geometry. Therefore, it is usually necessary to use methodsother than direct calculation to obtain an approximation of theacceleration at any point along end effector 12. For example, thedisplacement at each point may be a suitable approximation of theacceleration with a suitable scaling factor. Displacement may becalculated using, for example, finite element analysis of the blade.Alternatively, velocity at each point may be used to obtain an estimateof acceleration at a given frequency. The velocity at specific pointsmay be calculated by, for example, physically observing external pointsalong the blade surface, (e.g. using a laser vibrometer) and assumingthat the interior points are acting in the same manner as the surfacepoints. As another example, the velocity of any point along the blademay be approximated as substantially sinusoidal function of the distancefrom the balance node point.

The calculation of position vector {right arrow over (o)}(x, y, z) isgenerally tied to the method used to calculate {right arrow over (A)}(x,y, z). For example, if {right arrow over (A)}(x, y, z) is measured orapproximated at specific points along the end effector, then {rightarrow over (o)}(x, y, z) would be the position vector taken at thosespecific points.

Since ultrasonic instruments according to the present invention normallyutilize end effectors constructed of titanium, aluminum or an alloy oftitanium or aluminum the density at any point ρ(x, y, z) is a constant.Therefore, in general ρ(x, y, z)=P where P is the density of thematerial used in the end effector.

In practice, an end effector is designed which incorporates suitablebeneficial asymmetries into treatment region 26. Those beneficialasymmetries induce an undesirable torque {right arrow over (T)}_(u) atbalance point 29 which may be calculated using Equation (1). Once theundesirable torque {right arrow over (T)}_(u) for a particular design isknown, balance asymmetries may be added in balance region 28 to generatea balance torque {right arrow over (T)}_(b) at balance point 29 whichcancels the undesirable torque {right arrow over (T)}_(u) generated bythe beneficial asymmetries. Adding balance asymmetries to balance region28 consists of adding or subtracting mass from particular portions ofbalance region 28. The size and position of the mass added or subtractedis determined not only by the balance torque {right arrow over (T)}_(b)induced at balance point 29 but also by considerations such as theeffect upon the look, feel and ergonomics of the end effector.Therefore, once {right arrow over (T)}_(u) is calculated, the designermay begin to add and subtract mass from balance region 28 to create twoor more balance asymmetries which induce a beneficial torque at balancepoint 29.

It may be possible to simplify the calculations required, for example,using suitable assumptions, it is possible to simplify Equation (1) forthe purpose of calculating {right arrow over (T)}_(b). In particular, byassuming that the balance asymmetries can be modeled as a series ofpoint masses and neglecting the effect of rotation: $\begin{matrix}{{\overset{\rightharpoonup}{T}}_{b} = {\sum\limits_{n = 1}^{k}{m_{n}{\overset{\_}{\overset{\rightharpoonup}{A}}}_{s,n} \times {\overset{\rightharpoonup}{o}}_{{CM}_{m_{n}}}}}} & (2)\end{matrix}$

where:

m_(n) is the mass of the end effector at each point n;

{right arrow over (T)}_(b) is the torque induced at balance point 29 bythe balance asymmetries designed into balance region 26;

k is the total number of balance asymmetries;

{right arrow over ({overscore (A)})}_(s,n) is the average over asurface, or a representative vector acceleration at the point in balanceregion 26 where mass n is added; and

{right arrow over (o)}_(CM) _(mn) is an offset vector pointing to theCenter of Mass of mass n.

By designing the balance asymmetries to be symmetrical about plane ofsymmetry 50, the torque exerted at node 22 may be modeled as beingentirely about the y-axis of the end effector. If all balanceasymmetries are located on a plane of symmetry 50, equation (2) becomes:

{right arrow over (T)} _(b) =m ₁ {right arrow over ({overscore (A)})}_(s,1) ×{right arrow over (o)} _(CM) _(m1) +m ₂ {right arrow over({overscore (A)})} _(s,2) ×{right arrow over (o)} _(CM) _(m2) +m ₃{right arrow over ({overscore (A)})} _(s,3) ×{right arrow over (o)}_(CM) _(m3) + . . .   (3)

or

{right arrow over (T)} _(b) ·{right arrow over (j)}=m ₁ {right arrowover ({overscore (A)})} _(s,1) ×{right arrow over (o)} _(CM) _(m1)·{right arrow over (j)}+m ₂ {right arrow over ({overscore (A)})} _(s,2)×{right arrow over (o)} _(CM) _(m2) ·{right arrow over (j)}+m ₃ {rightarrow over ({overscore (A)})} _(s,3) ×{right arrow over (o)} _(CM) _(m3){right arrow over (j)}+ . . .   (4)

or, neglecting signs, $\begin{matrix}{{{\overset{\rightharpoonup}{T}}_{b}} = {{\sum\limits_{n = 1}^{k}{m_{n}{\overset{\_}{\overset{\rightharpoonup}{A}}}_{s,n} \times {\overset{\rightharpoonup}{o}}_{{CM}_{n_{1}}}}}}} & (5)\end{matrix}$

It will be apparent that a significant number of combinations of balanceasymmetry sizes and shapes may be used to generate an appropriate torque{right arrow over (T)}_(b) at balance node 29. Further, the size andshape of the particular balance asymmetries chosen will be a function ofthe material used to create those asymmetries. Therefore, the designeris normally left to select balance asymmetries which not only generatethe desired balance torque {right arrow over (T)}_(b), but meet otherdesign criteria as well. Thus, the actual design of appropriate balanceasymmetries becomes an iterative exercise, with the blade designerselecting preferred shapes and positions for the balance asymmetriesthen checking those shapes and positions using one of Equation (1), (2)or (5). The shape and size of the balance asymmetries may be adjusted asrequired to generate {right arrow over (T)}_(b).

An end effector according to the present invention may also be designedusing one or more empirical methods such as, for example, using modalanalysis. In the empirical methods, the end effector is designed withappropriate beneficial asymmetries included in treatment region 26 andbalance region 28 is modeled as a symmetric connector between thetreatment region and transmission waveguide 14. Since treatment region28 includes beneficial asymmetries (e.g. curved blade edges 36) withoutcorresponding balance asymmetries in balance region 28, this first passend effector will tend to be unbalanced. Once a first pass end effectoris developed, a suitable measurement of the torque generated at apreselected point, such as balance point 29, may be selected. Forexample, the balance ratio of peak lateral displacement to peaklongitudinal displacement as measured in the transmission waveguide. Thefirst pass end effector may then be numerically modeled and vibratedusing modal analysis or finite element analysis techniques. With thefirst pass numerical model driven at a suitable generator frequency(e.g. 55 kHz), it is possible, using, for example, finite elementanalysis programs to determine the ratio of peak lateral displacement topeak longitudinal displacement at selected points along the transmissionwaveguide. The end effector may then be balanced (i.e. the ratio of peaklateral displacement to peak longitudinal displacement reduced to lessthan 1:10) by adding or subtracting mass in the balance region. This is,of course, an iterative process which may be enhanced (i.e. feweriterations required) by the skill and experience of the designer.

A further empirical design technique involves designing a first pass endeffector in the manner set forth above. A physical model of the firstpass end effector is then built and tested by driving the input of thetransmission waveguide at a suitable generator frequency. The frequencyat which the end effector is driven may be referred to as the drivefrequency. With the first pass end effector driven at the drivefrequency, a suitable measurement of the torque generated at the balancenode may be selected, for example, the balance ratio can be measureddirectly from the transmission waveguide. The end effector may then bebalanced (i.e. the balance ratio reduced to less than 1:10) byphysically adding or subtracting mass in the balance region. This is, ofcourse, an iterative process which may be enhanced (i.e. feweriterations required) by the skill and experience of the designer. Nomatter the design method chosen, whether empirical or analytical, if itis an iterative process, the rougher the first approximation used, themore iterations will be necessary to arrive at balanced blade design.

As described herein, balance node 22 was selected as the proximal originof balance region 26 in order to provide clarity and to set forth aphysically definable point of reference which may be located on anytransmission waveguide, using either mathematical computation orphysical measurements. As it happens, using node 22 as the proximalorigin of balance region 26 is also beneficial in that it is believed tomake the mathematics set forth herein cleaner and more understandable.However, it should be recognized that using the present invention, theundesirable torque generated in the waveguide will be substantiallycanceled by the balance torque generated in the wave guide from a pointjust proximal to the proximal most balance asymmetry. For example, inthe embodiment of the invention illustrated in FIG. 2, the torque willconverge toward zero in the portion of the waveguide proximal to firstpredetermined point 42.

While the embodiments illustrated and described herein have beneficialasymmetries in only one direction, the present technique could be usedto balance blades having asymmetries in any two or more directions. Itwill be further be apparent that, in a surgical end effector designedaccording to the present invention, the center of mass of the endeffector may not be positioned on the central axis of the waveguide. Ablade according to the present invention may also be designed to includea single or multiple angle of curvature and to include blunt, square orsharp blade edges. A balanced ultrasonic blade designed according to thepresent invention may be used to perform many open and endoscopicsurgical procedures, including: internal mammary artery (IMA) takedownprocedures; removal or dissection of the radial artery; breast reductionand reconstruction; and hemorrhoid removal. Ultrasonic blades, accordingto the present invention, have multiple functions and may includemultiple features to facilitate those functions, for example, flats orblunt regions for configuration, sharp or dull edges and serrated bladeedges.

While preferred embodiments of the present invention have been shown anddescribed herein, it will be obvious to those skilled in the art thatsuch embodiments are provided by way of example only. Numerousvariations, changes, and substitutions will now occur to those skilledin the art without departing from the invention. Accordingly, it isintended that the invention be limited only by the spirit and scope ofthe appended claims.

What is claimed is:
 1. An end effector for use in an ultrasonic surgicalinstrument, wherein said end effector comprises: an ultrasonictransmission rod having a proximal end and a distal end; anultrasonically actuated blade attached to said distal end of saidtransmission rod, wherein said ultrasonically actuated blade comprises:a distal end; a proximal end connected to said transmission rod at alongitudinal vibratory node point; a treatment portion including atleast one functional asymmetry; and a balance portion including firstand second balance asymmetries wherein said balance portion extends fromsaid distal end of said ultrasonic transmission rod to a proximal end ofsaid treatment portion and said treatment portion extends from a distalend of said balance portion to said distal end of said blade, whereinsaid first and second balance asymmetries are positioned to countertorque created at said proximal end of said blade by said functionalasymmetry.
 2. An end effector according to claim 1, wherein said firstand second balance asymmetries are positioned such that transversevibrations in said ultrasonic transmission rod are substantially equalto zero.
 3. An end effector according to claim 1 wherein saidtransmission rod has a balance ratio of less than 1:10.
 4. An endeffector according to claim 3 wherein said balance ratio of saidtransmission rod is less than 1:200.
 5. An end effector for use in anultrasonic surgical instrument, wherein said end effector comprises: anultrasonic transmission rod having a proximal end and a distal end; abalanced ultrasonically actuated blade attached to said distal end ofsaid transmission rod, wherein said balanced ultrasonically actuatedblade comprises: a distal end; a proximal end; a curved treatmentportion; and a balance portion including first and second balanceasymmetries wherein said balance portion extends from said distal end ofsaid ultrasonic transmission rod to a proximal end of said treatmentportion and said treatment portion extends from a distal end of saidbalance portion to said distal end of said blade wherein said balanceasymmetries are positioned to counter torque created at said proximalend of said blade by said curved treatment portion.
 6. An end effectoraccording to claim 5 wherein said end effector has a top surface and abottom surface, said top surface being concave in said treatment portionand said bottom surface being convex in said treatment region, saidfirst balance asymmetry being positioned on said top surface and saidsecond balance asymmetry being positioned on said bottom surface.
 7. Anend effector according to claim 6 wherein said first balance asymmetrycomprises a first flat region on said top surface of said balanceportion.
 8. An end effector according to claim 7 wherein said secondbalance asymmetry comprises a second flat region on said bottom surfaceof said balance portion.
 9. An end effector according to claim 8 whereinsaid blade is bisected by a plane of symmetry, said blade beingsubstantially symmetrical on either side of said plane of symmetry, saidfirst balance asymmetry comprising a flat surface on said top surface ofsaid blade wherein said flat surface is substantially perpendicular tosaid plane of symmetry.
 10. An end effector according to claim 9 whereinsaid second balance asymmetry comprises a second flat surface on saidbottom surface of said blade, wherein said flat surface is substantiallyperpendicular to said plane of symmetry.
 11. An end effector accordingto claim 6 wherein at least one of said first and second balanceasymmetries comprises a notch in said balance region.
 12. An endeffector according to claim 6 wherein at least one of said first andsecond balance asymmetries comprises a raised region including massadded to said balance region.
 13. An ultrasonic surgical instrumentincluding a balanced end effector, wherein said end effector comprises:a handle including an ultrasonic handpiece; an ultrasonic transmissionrod having a proximal end and a distal end, wherein said proximal end isoperatively connected to said handpiece; an ultrasonically actuatedblade attached to said distal end of said transmission rod, wherein saidultrasonically actuated blade comprises: a distal end; a proximal endconnected to said transmission rod at a longitudinal vibratory nodepoint; a treatment portion including at least one functional asymmetry;and a balance portion including first and second balance asymmetrieswherein said balance portion extends from said distal end of saidultrasonic transmission rod to a proximal end of said treatment portionand said treatment portion extends from a distal end of said balanceportion to said distal end of said blade, wherein said first and secondbalance asymmetries are positioned to counter torque created at saidproximal end of said blade by said functional asymmetry.
 14. Anultrasonic surgical instrument according to claim 13, wherein said firstand second balance asymmetries are positioned such that transversevibrations in said ultrasonic transmission rod are substantially equalto zero.
 15. An end effector according to claim 13 wherein saidtransmission rod has a balance ratio of less than 1:10.
 16. An endeffector according to claim 15 wherein said balance ratio of saidtransmission rod is less than 1:200.
 17. An ultrasonic surgicalinstrument comprising: a handle; an ultrasonic transmission rod having aproximal end and a distal end wherein said proximal end is operativelyconnected to said handle; a balanced ultrasonically actuated bladeattached to said distal end of said transmission rod, wherein saidbalanced ultrasonically actuated blade comprises: a distal end; aproximal end; a curved treatment portion; and a balance portionincluding first and second balance asymmetries wherein said balanceportion extends from said distal end of said transmission rod to aproximal end of said treatment portion and said treatment portionextends from a distal end of said balance portion to said distal end ofsaid blade.
 18. An ultrasonic surgical instrument according to claim 17wherein said blade has a top surface and a bottom surface, said topsurface being concave in said treatment portion and said bottom surfacebeing convex in said treatment region, said first balance asymmetrybeing positioned on said top surface and said second balance asymmetrybeing positioned on said bottom surface.
 19. An ultrasonic surgicalinstrument according to claim 18 wherein said first balance asymmetrycomprises a first flat region on said top surface of said balanceportion.
 20. An ultrasonic surgical instrument according to claim 19wherein said second balance asymmetry comprises a second flat region onsaid bottom surface of said balance portion.
 21. An ultrasonic surgicalinstrument according to claim 18 wherein said blade is bisected by aplane of symmetry, said blade being substantially symmetrical on eitherside of said plane of symmetry, said first balance asymmetry comprisinga flat surface on said top surface of said blade wherein said flatsurface is substantially perpendicular to said plane of symmetry.
 22. Anultrasonic surgical instrument according to claim 21 wherein said secondbalance asymmetry comprises a second flat surface on said bottom surfaceof said blade, wherein said flat surface is substantially perpendicularto said plane of symmetry.
 23. An ultrasonic surgical instrumentaccording to claim 18 wherein at least one of said first and secondbalance asymmetries comprises a notch in said balance region.
 24. Anultrasonic surgical instrument according to claim 18 wherein at leastone of said first and second balance asymmetries comprises a raisedregion including mass added to said balance region.